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All,
Sithra (thanks Sithra!) was the first to alert us of this brand new study [1] out yesterday in Nature, now getting lots of attention in the popular press (e.g. here, here and here). He made relatively casual mention of it, deep in this thread on Intrinsic Aging, so at first I didn't realize its significance. Now that I do, I think it definitely deserves its own thread.
It could be the kind of "out of the blue" breakthrough mentioned here that just might rapidly advance the science of human longevity, or at least put us on the road to longevity escape velocity to give Aubrey and Co. time to solve the plethora of other problems (discussed here and here) which cause aging...
With that build-up, what was the study about, and what did they find?
It was a study in mice, and the effects on health & longevity of killing off of senescent cells - old dysfunctional cells that are no longer dividing, but that refuse to die and as a result spew out reactive oxygen species (ROSs) and inflammatory chemicals into the body. It has long been suspected that senescent cells contribute to aging, but it has been hard to prove it. It seems that the researchers in this study may have done that, and opened up new research directions for anti-aging therapy.
What they actually did is rather complicated. But in a nutshell, as I understand it, here is what they did. It seems that in addition to all other crap that senescent cells spew out, they generate a tumor suppressing protein called p16Ink4a, which I'll abbreviate as p16. From this popular press article:
You can think of [p16] as basically [the senescent cells'] calling card.
By rewriting a tiny portion of the mouse genetic code, Baker and van Deursen's team developed a genetic line of mice with cells that could, under the right circumstances, produce a powerful protein called caspase when they start secreting p16. Caspase acts essentially as a self-destruct button; when it's manufactured in a cell, that cell rapidly dies [via apoptosis].
So what exactly are these circumstances where the p16 secreting cells start to create caspase and self-destruct? Well, only in the presence of a specific medicine the scientists could give the mice. With their highly-specific genetic tweak, the scientists had created a drug-initiated killswitch for senescent cells.
So the researchers took mice genetically modified to carry this senescent cell "kill switch" and started injecting them with the kill switch activator at 12 months of age (around the human equivalent of 45 years old). This resulted in the death of a large fraction of senescent cells in various parts of the mice with the kill switch. As a result, in two strains of mice, both males and females median lifespan was significantly extended by about 25%. Here are the male-only survival curves of controls without the kill switch but treated with the activator (C57 +AP), controls with the kill switch but without activating it (ATTAC -AP) and the treatment group with the kill-switch which was activated (ATTAC +AP) to kill off the senescent cells, for the commonly-employed C57BL/6 strain of mice:
The magenta curve shows administering the activator alone doesn't improve or harm the survival of natural mice without the genetically-engineered kill-switch (C57 +AP). The dark blue (solid) curve shows the genetic modification, without the activator, doesn't improve or harm mice survival either (ATTAC -AP). The light blue (dashed) curve shows that in mice with the kill-switch and treated with the kill-switch activator (ATTAC +AP), lived significantly longer on average, by in this case, a whopping 35%. The "xxx d" numbers associated with each curve represent the different groups' median lifespan.
Now before we jump to any conclusions, we should do as Michael always says, and check the longevity of these mice against other studies of the same strain, and especially compare their longevity with the results of CR. From this study [2], discussed here, the median lifespan of male-only C57BL/6 is 26.3 months for AL fed mice and 32.6 months for CR fed mice. At an average of 30.4 days per month, that is a median lifespan of 800 days for AL mice, and 991 day for CR mice (24% life median life extension for CR).
Hmmm... That calls these results into question a bit. Why? Because well-cared-for C57BL/6 mice fed ad lib appear to live 800 days in other labs, whereas the equivalent so-called "controls" in this study lived only 626 days. Killing off the senescent cells eliminated this early death effect observed in the so-called controls, boosting the treated mice to a median lifespan of 843 days. But this 843 days is only marginally longer (if at all) than the median lifespan of well-cared-for ad lib controls in this strain (800 days), and nowhere near the median lifespan of male C57BL/6 mice subjected to CR (991 days).
The other important thing to notice is that in the above survival curve, the median lifespan of the treated mice was increased, but not the maximum lifespan. The two blue curves hit zero on the x-axis at the exact same age. This is in contrast with the effect of CR in this strain, where the median and maximum lifespan of CRed mice is greatly extended, as can be seen from the survival curve from [2]:
Whether it was due to poor animal husbandry, or something in the genetic manipulation, treatmetn and/or kill-switch activator compound itself, something was killing off the control animals early in this new study. The treatment appears to restore their median longevity to the natural median lifespan of well-cared-for ad lib-fed controls, but even there the maximum lifespan of well-cared-for ad lib-fed controls was 1042 days (34.3 months), while the treated animals in this study lived to a maximum of only 900 days. So the treated mice didn't even come close to the maximum lifespan of ad lib fed well-cared-for mice, to say nothing of the 1300 day maximum lifespan of the well-cared-for CR C57BL/6 mice.
This may explain several anomalies between the popular press reports of this study and the full text of the study itself. First of all, compare the gushing popular press headlines:
In New Anti-Aging Strategy, Clearing Out Old Cells Increases Life Span of Mice by 25 Percent - MIT Technology Review
AGEING BREAKTHROUGH: RESEARCHERS ADD UP TO A THIRD TO MICE’S LIFESPANS BY CLEARING OLD CELLS - Factor-tech.com
Scientists Can Now Radically Expand the Lifespan of Mice—and Humans May Be Next - Popular Mechanics
with the much more modest title from the Nature paper itself on which they are reporting:
Naturally occurring p16(Ink4a)-positive cells shorten healthy lifespan - Nature
I've underlined the key difference - "shortens" vs. "increases", "adds up to a third" or "radically expands". With the very title of their paper the authors are conceding that they haven't actually increased the median (to say nothing of maximum) lifespan of mice relative to normal, ad lib controls of the same strain with their treatment for killing off senescent cells. In the discussion section of the full text of the paper, the authors clarify what might be going on with this passage:
It will be useful to optimize senescent cell removal protocols and methods further because the longevity of male C57BL/6 mice seemed negatively affected by repetitive vehicle injection stress, and because clearance was partial and several key tissues were refractory to clearance, including liver and colon.
In short, so far the 'cure' (killing off senescent cells) seems worse (or at least not significantly better) than the 'disease' (living with senescent cells).
Plus, remember the mice had to be genetically engineered so that their p16-expressing senescent cells would be targeted by the apoptosis-activating compound, a genetic modification that a technology like CRISPR might one day be able to pull-off in humans, but that day is a long way off.
So not for the first time, I started off a post with a flourish of enthusiasm, only to discover the popular press has seriously overhyped the significance of the research. I considered going back and curbing the enthusiasm I expressed in the introductory paragraphs of this post. But I figured it was better not to - since this way it serves as a nice case-study in the value and importance of careful reading and analyzing the original source.
Overall, the results are certainly suggestive that senescent cells are bad news for health & longevity, but we pretty much knew that already. Getting rid of senescent cells may indeed extend longevity, at least on average. But true (maximum) human lifespan extension, still seems to remain a long way off... But this hasn't stopped the researchers involved from partnering with the Buck Institute to form a biotech startup called Unity Biotechnology to try to push towards commercializing methods to clear senescent cells.
--Dean
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[1] Nature. 2016 Feb 3. doi: 10.1038/nature16932. [Epub ahead of print]
Naturally occurring p16(Ink4a)-positive cells shorten healthy lifespan. Baker DJ(1), Childs BG(2), Durik M(1), Wijers ME(1), Sieben CJ(2), Zhong J(1), A Saltness R(1), Jeganathan KB(1), Verzosa GC(3), Pezeshki A(4), Khazaie K(4), Miller JD(3), van Deursen JM(1,)(2). Author information: (1)Department of Pediatric and Adolescent Medicine, Mayo Clinic College of Medicine, Rochester, Minnesota 55905, USA. (2)Department of Biochemistry and Molecular Biology, Mayo Clinic College of Medicine, Rochester, Minnesota 55905, USA. (3)Division of Cardiovascular Surgery, Mayo Clinic College of Medicine, Rochester, Minnesota 55905, USA. (4)Department of Immunology, Mayo Clinic College of Medicine, Rochester, Minnesota 55905, USA.
Full text: http://www.nature.com.sci-hub.io/nature/journal/vaop/ncurrent/full/nature16932.html
Cellular senescence, a stress-induced irreversible growth arrest often characterized by expression of p16(Ink4a) (encoded by the Ink4a/Arf locus, also known as Cdkn2a) and a distinctive secretory phenotype, prevents the proliferation of preneoplastic cells and has beneficial roles in tissue remodelling during embryogenesis and wound healing. Senescent cells accumulate in various tissues and organs over time, and have been speculated to have a role in ageing. To explore the physiological relevance and consequences of naturally occurring senescent cells, here we use a previously established transgene, INK-ATTAC, to induce apoptosis in p16(Ink4a)-expressing cells of wild-type mice by injection of AP20187 twice a week starting at one year of age. We show that compared to vehicle alone, AP20187 treatment extended median lifespan in both male and female mice of two distinct genetic backgrounds. The clearance of p16(Ink4a)-positive cells delayed tumorigenesis and attenuated age-related deterioration of several organs without apparent side effects, including kidney, heart and fat, where clearance preserved the functionality of glomeruli, cardio-protective KATP channels and adipocytes, respectively. Thus, p16(Ink4a)-positive cells that accumulate during adulthood negatively influence lifespan and promote age-dependent changes in several organs, and their therapeutic removal may be an attractive approach to extend healthy lifespan. PMID: 26840489
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[2] Genotype and age influence the effect of caloric intake on mortality in mice.
Forster MJ, Morris P, Sohal RS.
FASEB J. 2003 Apr;17(6):690-2. Epub 2003 Feb 5.
PMID: 12586746 Free PMC Article
http://www.ncbi.nlm....les/PMC2839882/
http://www.ncbi.nlm....nihms182370.pdf
Abstract
Long-term caloric restriction (CR) has been repeatedly shown to increase life span and delay the onset of age-associated pathologies in laboratory mice and rats. The purpose of the current study was to determine whether the CR-associated increase in life span occurs in all strains of mice or only in some genotypes and whether the effects of CR and ad libitum (AL) feeding on mortality accrue gradually or are rapidly inducible and reversible. In one experiment, groups of male C57BL/6, DBA/2, and B6D2F1 mice were fed AL or CR (60% of AL) diets beginning at 4 months of age until death. In the companion study, separate groups of mice were maintained chronically on AL or CR regimens until 7, 17, or 22–24 months of age, after which, half of each AL and CR group was switched to the opposite regimen for 11 wk. This procedure yielded four experimental groups for each genotype, namely AL==>AL, AL==>CR, CR==>CR, and CR==>AL, designated according to long-term and short-term caloric regimen, respectively. Long-term CR resulted in increased median and maximum life span in C57BL/6 and B6D2F1 mice but failed to affect either parameter in the DBA/2 mice. The shift from AL==>CR increased mortality in 17- and 24-month-old mice, whereas the shift from CR==>AL did not significantly affect mortality of any age group. Such increased risk of mortality following implementation of CR at older ages was evident in all three strains but was most dramatic in DBA/2 mice. Results of this study indicate that CR does not have beneficial effects in all strains of mice, and it increases rather than decreases mortality if initiated in advanced age.
Keywords: caloric restriction, aging, C57BL/6, DBA/2, B6D2F1